How would I go about creating a spiralled dome?
As usual, it looks as if this is simple enough to model, but the devil is in the details. Look closely at the flat space between each section, and notice how the twist is not entirely uniform either.

And I feel a non-destructive approach would be best, so that other variants of onion domes can be constructed fast. 

My rendition:


start with a curve. I used the "extra objects-->Curves Galore, and used the flower:


I made some adjustments to the overall curve, and added a second circle in the same object while in edit mode (later I adjusted to this final version - it is hard to predict the exact shape at this stage):


Then I added a straight bezier profile curve, and subdivided this curve (W menu) about 5~7 times to get the resolution required for the lattice deformer at a later stage.:


Next, simple deformers are added to create a base twist and tapering (which makes it much easier to control when applying the lattice later). A taper, and a twist:


Create a lattice object. I used W:8 divisions.


apply the lattice deformer to the curve object, and start tinkering around with the size. Key ingredient: rotate each lattice sub division to control the twisting precisely.
This offers much more control than the simple twist deformer, allowing for a visually more correct twist like the one seen in the original onion domes.
Mine is a bit messy, because I was a bit in a hurry:


Done! Completely non-destructive onion dome, with a lot of control over the final result.




It literally take 20 seconds to change the profile curve, and generate quite complex bizarre looking domes:

Download the example blend here:

Addendum: adding small circles in the base curve object creates very pronounced "ridges" as seen in the original: